A rotorcraft is provided with at least one engine that drives a main rotor for providing the aircraft with lift and even with propulsion. The rotor comprises a hub that is driven by a rotor shaft, in turn driven by the engine, and a plurality of blades are arranged on the hub.
While rotating, the blades are subjected to a torsor of forces, and consequently they are subjected to centrifugal force as well as to multiple effects due to flapping, due to drag, and due to twisting, where twisting is caused in particular by the changes in pitch that serve to change the inclination of the blades relative to the plane of the hub.
In addition, the hub generally includes a plurality of arms, that are preferably flexible in flapping, with the blades being arranged at the ends of those arms. The forces due in particular to centrifugal force are then transmitted to the non-flexible central zone of the hub.
Under such conditions, a first fastener device is known for fastening each blade to the hub that makes use of external means, in particular a sleeve. For example, it may be constituted by the configuration known to the person skilled in the art by the name “Starflex” as implemented in particular on “Ecureuil” helicopters, where the terms “Starflex” and “Ecureuil” are trademarks registered in the name of Eurocopter.
Under such circumstances, the hub has a solid and rigid central portion that is extended radially by as many arms that are flexible in flapping as there are blades, thus forming an assembly with the appearance of a star when seen from above.
At its root, each blade is rigidly secured, via its spar(s), to radially outer, first ends of two straps forming a sleeve, those straps being disposed on either side of the blade and the flexible arm. In addition, the inner, second radial ends of the two straps are secured to a laminated spherical abutment which is also secured to the flexible arm by being arranged in an opening present at the base of the flexible arm.
Consequently, the centrifugal force on each blade is taken up via the corresponding sleeve by the associated laminated spherical abutment, which transmits said force to the solid and rigid central portion of the hub.
Flapping, drag, and twisting are taken up firstly via the laminated spherical abutment, and secondly by adding a joint. The joint then comprises a self-lubricating ball joint fastened to the end of the flexible arm and secured to two viscoelastic elastomer soleplates, each connected to one of the two straps.
Although very effective, it will be understood that that first device is not necessarily optimized from an aerodynamic point of view, because of the presence of the straps which can lead to aerodynamic disturbances.
Document EP 0 448 685 discloses a second device for fastening a blade to a hub that implements external means, namely a cuff. A cuff merely constitutes a sleeve that is faired and a priori hollow. This definition for the term “cuff” is explained below.
The hub then has a passage at the base of each arm, with a laminated elastomer support secured to the hub being arranged in the passage. Similarly, the end of each arm is provided with a joint provided with an elastomer support.
Each blade is then fastened to one end of the cuff via its root, said cuff itself being secured to the joint and to the laminated elastomer support. In addition, the cuff surrounds the arm of the hub, thereby increasing the aerodynamic performance of the assembly because of the fairing of the blade-cuff assembly.
Nevertheless, since the cuff is then constituted by means on the outside of the blade, the use of a cuff requires an interface enabling the blade to be secured to the cuff.
Unfortunately, such an interface is penalizing in terms of weight. This has multiple consequences. Since the blade-and-cuff assembly is naturally heavier, the forces to which it the assembly is subjected and that are transmitted to the hub are greater, thus implying that the hub needs to be reinforced. Consequently, the hub is more voluminous and not very streamlined. It can thus be understood that the hub becomes particularly bulky, and overall the performance of the rotor is diminished.
To remedy the above drawbacks, document EP 0 085 129 discloses a third device in which the cuff is integral with the blade.
The cuff is then constituted by a casing that is rigid in twisting and that extends the casing of the blade. In addition, the cuff surrounds, without touching, a segment of the neck of the blade that is fastened to the hub of the rotor.
Consequently, the cuff forms part of the blade, in particular by being constituted by the covering thereof. It is also connected to a rod so as to be able to control the pitch of the blade.
By integrating the cuff in the blade, that third device eliminates the system for fastening the blade to the cuff that is provided in the second device, thereby achieving a corresponding overall saving in weight.
However, it is found that the cuff presents a section that is circular, or even nearly rectangular, which is not suitable for obtaining good aerodynamic performance. Similarly, the cuff is used ultimately only for transmitting twisting to the blade (controlling pitch), with the flapping, drag, and centrifugal forces being taken up by the neck segment of the blade. It can be understood that that cuff constitutes weight that is practically dead since its use is confined to controlling the pitch of the blade.